Multigrade lubricating oils employed in truck and passenger car internal combustion engines typically contain several types of additives, each imparting one or more essential properties to the oil. Dispersant additives are employed in lubricating oils to disperse and suspend sludge that is formed during engine operation. One widely used type of dispersant additive consists of a long chain hydrocarbon polymer chemically linked to a polar component, and is exemplified by polyalkenyl succinic anhydrides and their nitrogen-containing and ester derivatives. In particular, succinimide compositions obtained by reaction of polyamines with polyalkenyl succinic anhydrides have been commonly used as dispersant additives.
Polyalkenyl succinic anhydrides have typically been prepared by reacting a polyalkene having a number average molecular weight ("M.sub.n ") of from about 500 to about 5,000 with an unsaturated dicarboxylic compound (typically maleic anhydride) at elevated temperature (the thermal ene process) or at elevated temperature in the presence of chlorine (the chloro process). The polyalkene reactant of choice has often been a conventional polyisobutene obtained from butene streams by cationic polymerization in the presence of an aluminum chloride catalyst. Exemplary thermal ene processes are described in U.S. Pat. No. 3,361,673 and U.S. Pat. No. 3,401,118, and exemplary chloro processes are described in U.S. Pat. No. 3,172,892, U.S. Pat. No. 3,215,707, EP-A-382450 and GB-A-1440219.
Ethylene .alpha.-olefin polymer substituted succinic anhydrides have also been disclosed as suitable intermediates for preparing dispersant additives. U.S. Pat. No. 5,229,022, for example, discloses nitrogen-containing and ester dispersants derived from EAO polymer-substituted succinic anhydrides that have been prepared via the thermal ene process and via the chloro process from EAO polymers with a M.sub.n of about 300 to about 20,000 and having a high degree of terminal vinylidene unsaturation.
The polyalkenyl and EAO polymer-based succinic anhydride products resulting from thermal ene and chloro processes typically contain a functionalized (i.e., succinated) hydrocarbon polymer that includes monofunctionalized (i.e., containing one succinic group) and polyfunctionalized (i.e., containing two or more succinic groups) polymer molecules. The thermal ene products are characterized by having a carbon-carbon double bond, and the chloro products are characterized by having a carbon-carbon double bond or a ring other than a succinic ring, and residual chlorine.
Succinic-containing polymers suitable as dispersants per se and as dispersant intermediates have also been prepared by reacting unsaturated dicarboxylic compounds with hydrocarbon polymers in the presence of free radical initiators. U.S. Pat. No. 5,112,507 discloses copolymers having alternating succinic and polyalkyl groups prepared by reacting polyalkenes (especially polyisobutenes) having a high proportion of their unsaturation as alkylvinylidene unsaturation with unsaturated acidic reactants (especially maleic anhydride) in the presence of a free radical initiator. US '507 further discloses polysuccinimides prepared by reacting the alternating copolymer with polyamine. U.S. Pat. No. 5,578,237 discloses grafting EAO polymers of M.sub.n of about 700 to about 10,000 and having a high degree of terminal vinylidene unsaturation with mono-unsaturated carboxylic reactants such as maleic anhydride in the presence of a free radical initiator to obtain a product composed of EAO polymers having carboxylic groups such as succinic groups randomly attached along the polymer chains. US '237 further discloses gel-free nitrogen-containing and ester dispersants prepared by reacting the grafted EAO polymers with nucleophilic reactants selected from amines, alcohols, and/or amino alcohols, wherein the reaction is conducted in the presence of a chain-stopper when the nucleophilic reactant has more than one reactive amino or hydroxy group per molecule, in order to ensure a gel-free product.
Multifunctional viscosity index ("VI") improvers are incorporated into lubricating oils to reduce the tendency of the oils to change viscosity with changes in temperature during engine operation, while at the same time performing one or more other desirable functions such as dispersancy. Ethylene copolymers (e.g., ethylene-propylene binary copolymers and ethylene-propylene-diene terpolymers) with M.sub.n 's of from about 20,000 to about 300,000 have been grafted with carboxylic monomers, typically maleic anhydride, in the presence of a free radical initiator, and the grafted products subsequently reacted with amines to provide products suitable for use as dispersant-VI improvers. Exemplary dispersant-VI improvers based upon ethylene copolymers are described in U.S. Pat. No. 4,160,739, U.S. Pat. No. 4,171,273, and U.S. Pat. No. 4,505,834.
Despite the fact that a wide variety of materials have been disclosed to be useful as dispersants and multifunctional VI improvers, the need still exists for new materials which can provide improved performance in internal combustion engines, particularly in view of the increasingly stringent and demanding performance standards being placed upon engine oils by the American Petroleum Institute and engine manufacturers. Additives with improved dispersancy are needed, as are additives which can provide lubricating oils with a good balance between high- and low-temperature viscometric performance. The achievement of a proper balance between low and high temperature viscometric engine performance is an ongoing problem. Relatively high oil viscosities are needed at high engine operating temperatures to keep the engine surfaces properly lubricated. On the other hand, relatively low viscosities are required at low operating temperatures for efficient oil flow in the engine. At low operating temperatures such as those encountered during engine start-up, the oil can contain wax crystals, whose presence can make it difficult to crank the engine without wear damage to engine surfaces and can also impede proper circulation of the oil-wax slurry in the engine.